Crude vegetable glyceride oils, as they are obtained from their natural sources by conventional extraction methods, normally contain various non-glyceride impurities. These nonglyceride substances include gross material from the source of fat, such as xanthophyll or chlorophyll; products obtained by the breakdown of the glyceride during treatment, such as free fatty acids; and other derivatives of the glycerides, such as phosphatides and sterols. In addition, many vegetable oils normally contain natural waxes from the crushing of the coat of the oilseeds employed. Some of these above-mentioned impurities are desirable in that they help to protect the oil from oxidation or other adverse processes, but by far the greater amount of these substances must be removed during processing for edible purposes because they are deleterious to the appearance, taste, keeping qualities, and other properties of the oil. Refining and winterizing or dewaxing operations have, thus, become commonly employed to effect the removal of these impurities.
The removal of gross impurities, gummy or mucilaginous material, and the free fatty acids from the glyceride oil is commonly referred to as "refining" and as herein used the term excludes "bleaching" (color removal) and odor removal. In a typical refining operation, undesirable impurities are preferentially combined with a refining agent to form hydrophilic components. These are subsequently removed from the oil by a separation of aqueous and oil phases. Known methods of refining include contacting the glyceride oil with strong or dilute alkaline materials followed by separation of impurities, by liquid-liquid extraction of impurities from the glyceride oils, by steam distillation, and by contacting the glyceride oils with acids. Each of these methods is said to have its advantages for use in refining oils of one type or another for a certain ultimate utility by removing to a greater or lesser extent the hydrophilic component of the oils.
However, these known refining methods do not remove all of the impurities from the vegetable oil, and in particular, waxy components tend to remain in such oils. For example, if the refined vegetable oils is cooled to a temperature of about 40.degree.F, the higher melting triglycerides and any vegetable waxes (linear esters) present will crystallize and either impart a cloudy appearance to the oil or settle out as a crystalline precipitate. When the oil is again raised to room temperature, the crystallized waxes may redissolve in the oil. Thus, the oil at room temperature may or may not regain its clarity depending upon the amount of the respective impurities contained in it. Thus, without further processing, any such vegetable oil containing these higher melting triglycerides or vegetable waxes is not suitable for certain purposes where the clarity of the oil at low temperatures is important.
For example, oils which are suitable for salad oil use frequently are stored in refrigerators. The prolonged cooling of such oils to temperatures normally encountered in refrigerators, such as from about 30.degree.F to about 50.degree.F, requires a product which retains its clarity if it is to be desirable to the consumer.
The term "winterizing" has variously been applied to both the process of removing the higher melting triglycerides and the process of removing the naturally occurring vegetable waxes. To avoid this ambiguity in the term's usage, the term "winterization" will herein be used to refer only to the removal of the higher triglycerides from the oil and the term "dewaxing" will herein be employed to refer to the removal of naturally occurring vegetable waxes.
The ultimate objective of a refining or dewaxing operation is to remove every undesirable impurity completely, while at the same time maintaining intact all of the desirable glyceride oil. The particular process used with a given oil is determined by the foregoing considerations of maximum impurity removal with the minimum of glyceride oil loss. Since a good part of the refining cost arises from losses of glyceride oil, much work has been done to increase the efficiency of refining and dewaxing operations, and many processes have been developed for this purpose. The majority of the refining processes developed employ temperatures of at least room temperature and often higher to obtain a complete removal of the hydrophilic impurities and to minimize oil losses. These processes, of course, do not accomplish removal of the waxy component, which is inseparable from the glyceride component at these higher temperatures. Thus, a separate low-temperature dewaxing step is necessary to remove the waxy component. Since there are oil losses inherent in the separation steps which usually follow the refining and dewaxing operations, methods which embody multiple separation steps tend to be uneconomic. Low-temperature refining methods have been attempted to simultaneously remove the hydrophilic and waxy components from the crude oil; however, the methods developed thus far have not been entirely satisfactory. At low temperatures, a virtually inseparable emulsion tends to be formed from a vegetable oil and an aqueous refining agent. This results either in extraordinarily high oil refining losses or an incomplete removal of the impurities, the latter which results in a cloudy oil at low temperatures.
Today, much of the glyceride oil is refined in a continuous process. This process involves the steps of bringing the oil and alkali to an elevated temperature, mixing these two materials, adjusting the temperature, if necessary, providing a sufficient hold time, subjecting the mixture to degasification or other steps as are necessary, and continuously separating the refined glyceride oil from the impurities by centrifugation. Thereafter, if dewaxing is necessary, the refined oil is cooled to a low temperature to crystallize the waxy components. These are then removed by either a slow filtration or a second aqueous separation step performed on the cooled oil. Cumulative oil losses result from the individual separation steps employed.